To predict the performance of UV systems for disinfecting wastewater a full scale UV system was subjected to changes in wastewater quality. The effect of changes in the UV transmission and suspended solids on fecal coliforms was measured. The level of UV transmission and suspended solids which can be treated is dependent upon the disinfection limit. There was no relationship between the UV transmission and the suspended solids and the degree of photoreactivation. An antibiotic labeled Escherichia coli was irradiated with the UV system and then observed in glass bottles and in the receiving stream for photoreactivation. Photoreactivation was observed in the glass bottles but not in the receiving stream. In the natural environment photoreactivation may not be important
A simplified computational model of the ultraviolet (UV) disinfection dose delivered to wastewater was developed and the model outputs were compared with pilot-scale biodosimetry data. The model assumed plug flow, an assumption that may apply for some open channel UV reactors designed for wastewater disinfection and in which case may eliminate the need for involved and expensive computational fluid dynamics analyses of flow patterns. The reactor residence times derived from this assumption were combined with the output from a two-dimensional, point-source summation UV irradiance model to calculate UV dose. The output from the irradiance model was adapted to account for the cross-sectional distribution of UV irradiance within the reactor. Two UV reactor configurations were modeled and pilot-tested at two UV transmittance (UVT) values that are typical for wastewater. The simplified model predicted doses that fell within the range of the observed UV doses under some flow and UVT conditions, however it did so inconsistently and over-predicted the doses at the highest tested flow rates. This was concluded to be due to a breakdown of the simplified plug flow assumption due to deviations in the velocity field distribution at higher flows. Therefore, while this modeling approach may provide "back of the envelope" initial estimates of the UV doses supplied to wastewater and may allow a qualitative evaluation of the effect of adjusting UV reactor design parameters (e.g., channel width, lamp spacing) on the resulting UV dose, the precise quantitative prediction of UV dose must continue to rely on more sophisticated models. Key words: ultraviolet disinfection, computational modeling, wastewater treatment.
The treatment objective of an ultraviolet disinfection system used in a wastewater application is to protect the aquatic environment and users of water that receive discharges from wastewater treatment plants. To ensure this objective is adequately met it is important to validate, or verify equipment performance for a specific application. The widely accepted method for completing this validation is by determining the UV dose delivery performance using biodosimetry. Until now a standard bioassay did not exist for UV equipment that is used to disinfect treated wastewater from a biological treatment plant that is achieving an average effluent quality of less than 30 mg/L BOD/TSS and disinfection requirements of 126 cfu/100 mL E. coli over a 30 day geometric mean or 200 cfu/100 mL fecal coliforms over a 30 day geometric mean. A uniform bioassay for wastewater UV equipment that can be widely adopted by industry and regulatory bodies was developed and approved by the International Ultraviolet Association. This bioassay for UV equipment is described.
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